A. Field of the Invention
The present invention relates to lithographic printing, and in particular to plate constructions imageable by ablative discharge as well as to polymeric silicone coating formulations useful in connection therewith.
B. Description of the Related Art
Polyorganosiloxane compounds, or "silicones", can be synthesized in a wide variety of forms, and are utilized in numerous commercial applications. Silicone compounds are based on the repeating diorganosiloxane unit (--R.sub.2 SiO--).sub.n, where R is an organic radical and n denotes the number of units in the polymer chain. Each end of the linear chain is terminated with a functional or non-functional end group; the chain may also be "branched" so as to deviate from a strictly linear structure.
The physical properties of a particular silicone formulation depend on the length of the polymer chain, the nature of the organic functional groups bonded to the silicon atoms, and the terminal groups (more precisely, the alpha and omega groups) at each end of the chain. For example, the most common silicone compounds are based on the polydimethylsiloxane unit, --Si(CH.sub.3).sub.2 O--, which, due to the relatively small organic content of the chains, have a limited range of compatibility with organic compounds. By contrast, silicones containing aryl functional groups tend to exhibit properties more commonly associated with organic materials, and such silicones are generally miscible with a broader range of such materials.
Many polymeric silicone compounds exhibit rubber-like characteristics. These compounds are employed in numerous industrial and commercial environments as a direct substitute for natural rubber. "Silicone rubber" is typically prepared from "silicone gum", which connotes a viscous, high-molecular-weight polydiorganosiloxane compound, by cross-linking (or "curing") the polymer chains. It is often convenient to form silicone rubber in situ on a surface by applying components that will form the rubber and subsequently curing these. Curing (or cross-linking) permanently immobilizes the polymeric components of the rubber by establishing chemical linkages between them.
Curing can be accomplished in a number of ways, but generally depends on the presence of reactive functional groups on the polymer chains that interact and bond with one another. "Condensation cure" reactions refer to those in which a small molecule is eliminated when the two functional groups are joined. Typical condensation-cure reactions in silicone chemistry involve combination of silanol functional groups with other such groups to produce an oxygen linkage with the elimination of water. "Addition cure" reactions involve no loss of species, and can involve, for example, hydrosilylation reactions between olefinic functional groups (such as vinyl) and hydrosiloxane groups.
Variations on the traditional condensation-cure reaction include the "moisture-cure" approach, in which a precursor functional group is first hydrolyzed to form a reactive hydroxyl-bearing group, which then combines with a silanol group as discussed above. Suitable precursor compounds include acetoxy, alkoxy and ketoxime functional silanes, which form acid, alcohol and ketoxime byproducts, respectively, upon hydrolysis.
Silanol-functional silicones and mixtures of silanol-functional silicones with silicones containing acetoxy, alkoxy or ketoxime groups are relatively stable so long as moisture is excluded; this is particularly true for silicone polymers having appreciable molecular weights. Obtaining useful reaction rates generally requires a catalyst such as a metal carboxylate compound.
Silanol groups also react with hydrosiloxane species to liberate hydrogen and produce the silicon-oxygen-silicon linkage characteristic of the condensation cures. Use of a metal salt catalyst (such as dibutyltindiacetate) is generally necessary to achieve useful reaction rates. Because it proceeds rapidly when catalyzed, this reaction is widely used for silicone coating formulations applied on a coating line to a web substrate.
In addition to these mechanisms, silicone polymers are sometimes cross-linked using multifunctional acrylate or methacrylate monomers. The polymers are exposed to an electron beam or combined with a photoinitiator species and then exposed to actinic radiation in order to produce free-radical derivatives that combine with one another. Other approaches to cross-linking are described in U.S. Pat. No. 4,179,295, the entire disclosure of which is hereby incorporated by reference.
The reactive, cross-linking functional groups can be incorporated at the termini of a polymer chain, or at a desired frequency within a copolymer chain. In order to achieve the elastomeric properties associated with silicone rubber, large polymeric units ("base polymers") are cross-linked by smaller oligomers or multifunctional monomers. Frequently, this is accomplished by providing the base polymers with one type of functional group, and incorporating the complementary functional group on the cross-linking molecules. For example, the addition-cure reaction described above can be utilized to produce elastomeric compositions by combining base polymer or copolymer chains that contain olefinic functional groups with small cross-linking molecules that have hydrosiloxane-functional termini.
Silicone rubber coatings have been adopted by some manufacturers of planographic printing plates. Planographic printing, as contrasted with letter-press and gravure printing, relies on plate constructions in which image and non-image areas lie substantially in the same plane. The plate is prepared by altering the affinities of different areas of the plate for ink. Depending on the type of plate imaging system employed, non-image plate areas become (or remain) oleophobic, or ink-repellent, while image areas remain (or become) oleophilic, or ink-accepting. Ink applied to the plate surface, e.g., by a roller, will adhere to the oleophilic image areas but not the oleophobic non-image areas. The inked plate is then applied to the recording medium (in direct printing) or to an intermediate "blanket" cylinder which then transfers the image to the recording medium (in offset printing).
Manufacturers of planographic printing plates often employ silicone rubber compositions as plate coatings due to their oleophobic character, which provides compatibility with conventional planographic printing techniques. Silicone coatings are commonly used in conjunction with so-called "dry" plates. In contrast to the traditional "wet" plate, which requires application of a fountain or dampening solution to the plate prior to inking in order to prevent ink from adhering to and transferring from non-image areas, the non-image material of dry plates is itself sufficiently ink-repellent that no fountain solution is necessary.
One hypothesis explains this effect as arising from interaction between the non-image component of a dry plate and the (usually aliphatic) solvent or solvents employed in printing inks, resulting in the formation of a thin layer of solvent on the surface of the non-image component. Like the fountain solution of a wet plate, this surface layer acts as a boundary and prevents the ink from adhering to the plate.
Blank dry plates are subjected to an imaging process that removes the silicone coating from image areas to reveal an oleophilic surface. Imaging can be accomplished in a number of ways. Photosensitization methods rely on incorporation of a photoresist material in the plate structure which, upon exposure to radiation (e.g., visible light), alters the solubility or anchorage properties of the silicone. In typical commercial plates, exposure to light results either in firm anchorage of the silicone coating to the plate (in positive-working plates) or in destruction of the existing anchorage (in negative-working plates). Depending on the process chosen, the plate is first exposed to actinic radiation passing through a positive or negative rendition of the desired image that selectively blocks transmission of the radiation to the plate. After this exposure step, the plate is developed in chemical solvents that either anchor the exposed silicone or remove it to produce the final, imaged plate.
For example, a number of photosensitive dry-plate constructions are currently known and used in the art. In one approach, the photosensitive material is combined with the silicone coating prior to its application onto a substrate. Another construction relies on incorporation of the photosensitive compound within an underlying layer, exposure either weakening or strengthening the bond between layers. See, e.g., U.S. Pat. Nos. 3,511,178 and 4,259,905. In a third alternative, ink-accepting toner particles are fixed to the silicone surface according to the desired image pattern.
Plates can also be imaged by means other than photoexposure, e.g., by ablation using spark-discharge apparatus (such as that described in U.S. Pat. No. 4,911,075, the entire disclosure of which is hereby incorporated by reference) or laser-discharge apparatus (such as that described in Ser. No. 07/917,481, the entire disclosure of which is hereby incorporated by reference) that utilize electronic signals to locate and produce a discharge at the precise positions on the plate where the silicone coating is to be removed to reveal an underlying oleophilic surface. The spark-discharge apparatus can make contact with the plate or be held at a relatively fixed distance above the plate during the imaging process.
Silicone compositions used as coatings for planographic printing plates typically include two basic constituents, namely, a primary polyorganosiloxane base-polymer component and a smaller cross-linking component. The base component is usually a linear, predominantly polydimethylsiloxane copolymer or terpolymer containing unsaturated groups (e.g., vinyl) or silanol groups as reactive centers for bonding with the cross-linking molecules. These groups are commonly situated at the chain termini; alternatively, it is possible to utilize a copolymer incorporating the reactive groups within the chain, or branched structures terminating with the reactive groups. It is also possible to combine linear difunctional polymers with copolymers and/or branch polymers. See, e.g., published Japanese Patent Applications 1-118843 and 1-179047.
The cross-linking component is generally a multifunctional, monomeric or oligomeric compound of low molecular weight, which is reacted with the first component to create connections among the chains thereof. The curing reaction generally requires some type of catalyst, either chemical or physical, to produce favorable kinetics. Platinum metal complexes (such as chloroplatinic acid) are often employed to facilitate addition cures, while metal salt catalysts (such as a dialkyltindicarboxylate) are frequently used in conjunction with condensation cures.
If the functional groups of the cross-linking component are situated at the chain termini, cross-linking molecules will form bridges among the base-polymer molecules (particularly if the latter have functional groups distributed along the chains). On the other hand, if the cross-linking component contains functional groups distributed along its length, each molecule will form numerous points of attachment with the base-polymer molecules. Typically, this type of cross-linking molecule is combined with base polymers having chain-terminal functional groups in order to maximize the number of different base-polymer molecules attached to each cross-linking chain.
Modifiers can be added to alter physical properties, such as adhesion or rheology, of the finished coating. One can also add colorants, in the form of dyes or pigments, to the silicone formulation to facilitate quality-assurance evaluation or monitoring of the photoexposure process. Pigments and/or dyes can also be used to enhance imaging performance in plates that will be imaged by ablation using discharge apparatus, as further described herein.
Current silicone coating formulations suffer from a number of disadvantages, some stemming from physical characteristics of the polymer system itself, and others arising from the requirements of available coating apparatus. Silicone coatings are generally prepared by combining a silicone polymer with a solvent (usually aliphatic) and, possibly, other volatile components to control viscosity and assist in deposition. Because the solvent evaporates after the coating is applied, the amount of silicone actually deposited per unit surface area depends on the relative proportions of silicone and volatile components; this proportion is referred to as the "solids content" of the composition, and is typically expressed as a percentage. Too little solvent can produce a coating that is difficult to apply, while excessive proportions of solvent can result in deposition of too little of the actual silicone material during coating, thereby reducing plate durability. The relatively low-weight polymers currently employed for producing printing plates tend to exhibit low solution viscosities; because of the necessity of preserving a minimum viscosity level for coating purposes, this characteristic limits the extent to which low-weight silicones can be diluted with solvent to control the deposition rate. Furthermore, these coatings also tend to require a narrow range of solids content for uniform application, a constraint that results in further limitation of the ability to vary dilution.
Finally, low-weight silicone compositions form poor dispersions with solid particles. For a growing number of platemaking applications, introduction of pigments or other particles is necessary for optimum plate performance. Not only is it difficult initially to disperse particles in low-weight silicones, but over time the particles that have, in fact, been dispersed tend to reagglomerate. Moreover, the above-noted problems involving low solution viscosities and narrow solids-content requirements become accentuated when particles are introduced into low-weight compositions.
A silicone coating composition is applied to a plate substrate using any of a variety of well-known coating techniques. The choice of technique is critical not only to the ultimate performance of the plate, but also to the efficiency and reliability of the overall platemaking process. Typical coating techniques include roll coating, reverse-roll coating, gravure coating, offset-gravure coating, and wire-wound rod coating. The coating procedure must be rapid enough to achieve a satisfactory production rate, yet produce a highly uniform, smooth, level coating on the plate. Even small deviations in coating uniformity can adversely affect plate performance, since the planographic printing process depends strongly on coplanarity of image and non-image areas; in other words, the printing pattern reproduced by the plate must reflect the configuration of oleophilic and oleophobic areas impressed thereon, and remain uninfluenced by topological characteristics of the plate surface.
Because the physical properties of a given silicone formulation can be varied only to a limited extent by the use of solvents and modifiers (especially in the case of low-weight silicones), particular coating formulations tend to favor use in conjunction with a particular type of coating line. For example, addition-cure coatings having 100% solids content are most advantageously applied using offset gravure-type coating equipment. For formulations having low viscosities (which typically imply low solids contents), roll-coating and rod-coating applications are preferred.
However, as a practical matter, the number of coating lines available to a particular manufacturer is likely to be limited. It may therefore prove impossible to utilize a particular silicone formulation with readily available coating technology, forcing plate manufacturers to choose formulations based on compatibility with their coating lines rather than optimal performance for a given application.
This limitation can prove appreciable, since coating properties required for a particular application may narrow the range of acceptable formulations. Some coating properties, such as durability, can depend not only on the silicone formulation or solids content, but also on the surface to which the coating is applied and the environment in which it is used. Other characteristics, such as the ability of the silicone matrix to accept and retain dispersions of large amounts of particulate material (a particularly important feature for spark-discharge planographic applications), can rule out entire classes of formulations and/or severely limit the number of coating techniques that may be employed.